1. Field of the Invention
The present invention relates to an ignition coil for use in an internal combustion engine, more particularly to an ignition coil having a columnar member with a plurality of magnetic plates stacked one on the other, and a primary winding and a secondary winding wound around the columnar member.
2. Description of the Related Arts
Recently, an ignition coil having a columnar configuration has been proposed so as to place the ignition coil within a cylindrical plug hole formed in an internal combustion engine. In Japanese Patent Laid-open Publication No. 4-87311, for example, there is disclosed an ignition coil having a central core assembled by a plurality of wires made of magnetic material to form a columnar configuration. In that publication disclosed are various methods for producing the central core assembled by the wires, such as a method for drawing the wires through a cylindrical die, a method for placing the wires between an upper die and a lower die, and then pressing them to form the core, a method for aligning the wires and pressing them in dies together with metallic powder, with resin powder adhered thereto, a method for placing the wires within a silicon steel pipe and then drawing the pipe through a die while heating them, and a method for rolling a silicon steel plate double or triple and then drawing it through a die. In that publication, also disclosed is a central core which is assembled by a plurality of wires to form opposite end portions having a rectangular cross section and a middle portion having a circular cross section.
As disclosed in the Publication No. 4-87311, when the ignition coil is to be formed in a cylindrical shape, it is desirable to form the central core or inner core also in the cylindrical shape. However, the methods for assembling a plurality of wires as described above can not be easily made, so that any of the methods will result in increase in cost.
Supposing that a conventional process for stacking a plurality of steel plates is employed to form a columnar core, the core will be formed as shown in FIGS. 13-16. That is, a plurality of silicon steel plates MPx having different width from one another are formed as shown in FIGS. 13-16. A recess is formed on one side of each steel plate MPx, and a protrusion is formed on the other side of each steel plate MPx. The silicon steel plates MPx are stacked one on the other under pressure to form an inner core 10x having stacked portions Stb. This "stacking process" is a process for forming a recess on one side, e.g., under side of each of a plurality of flat plates and a protrusion on the other side, e.g., upper side of each of the plates, when the plates are stamped, and then stacking the plates one on the other with each protrusion press-fitted into each recess mating with the protrusion to form a plurality of stacked portions on the plates.
According to the above process, however, if a columnar inner core having a diameter of e.g., 8 mm, is formed by the above process, the width of the outermost plate will be as small as 2 mm, as can be seen in FIG. 13. Therefore, if a plurality of steel plates are connected one another by the stacking process, the diameter of the stacked portion will be small in diameter to have a connecting strength of smaller than 2 kgf, so that the stacked plates will be easily peeled off. In the case where the columnar inner core is produced, the wider steel plate is stacked on the narrower steel plate in sequence to broaden the width of the stacked plates gradually, and then the narrower steel plate is stacked on the wider steel plate in sequence to narrow the width of the stacked plates gradually. In this case, however, if the wider steel plate is stacked on the narrower steel plate, the staked plates will be likely bent along the longitudinal direction, so that the stacking process will be difficult. As a result, a stamping process using a progressive die can not be employed, so that the productivity will be reduced and the manufacturing cost will be raised.